1. Field of the Invention
This invention relates generally to implantable medical devices and, more particularly, to methods, apparatus, and systems for stimulating a cranial nerve of a patient to treat a medical condition, such as metabolic disorders and/or hormonal imbalance associated with the thyroid gland.
2. Description of the Related Art
The human nervous system (HNS) includes the brain and the spinal cord, collectively known as the central nervous system (CNS). The central nervous system comprises nerve fibers. The network of nerves in the remaining portions of the human body forms the peripheral nervous system (PNS). Some peripheral nerves, known as cranial nerves, connect directly to the brain to control various brain functions, such as vision, eye movement, hearing, facial movement, and feeling. Another system of peripheral nerves, known as the autonomic nervous system (ANS), controls blood vessel diameter, intestinal movements, and actions of many internal organs. Autonomic functions including blood pressure, body temperature, heartbeat and essentially all the unconscious activities that occur without voluntary control.
Like the rest of the human nervous system, nerve signals travel up and down the peripheral nerves, which link the brain to the rest of the human body. Nerve tracts or pathways, in the brain and the peripheral nerves are sheathed in a covering called myelin. The myelin sheath insulates electrical pulses traveling along the nerves. A nerve bundle may comprise up to 100,000 or more individual nerve fibers of different types, including larger diameter A and B fibers which comprise a myelin sheath and C fibers which have a much smaller diameter and are unmyelinated. Different types of nerve fibers, among other things, comprise different sizes, conduction velocities, stimulation thresholds, and myelination status (i.e., myelinated or unmyelinated).
The human body includes many endocrine glands such as the pituitary gland, the pancreas, the adrenal glands, the parathyroid gland, and the thyroid gland. The thyroid gland is located in the lower part of the neck and is shaped in a butterfly structure. The thyroid gland comprises two separate lobes that are inter-connected via thyroid tissue. The two lobes are located on the opposite sides of the windpipe called the trachea.
A number of cranial nerves innervate the vicinity of a person's larynx. For example, the thyroid gland is innervated by the vagus nerve (cranial nerve X). The trigeminal nerve (cranial nerve V), the glossopharyngeal nerve (cranial nerve IX), and the vagus nerve (cranial nerve X) provide branches to the pharyngeal plexus, which innervates muscles and mucosa of the pharynx and many of the muscles of the soft palate. The vagus nerve uses an innervating branch called inferior thyroid nerve originating from the recurrent laryngeal nerve. The recurrent laryngeal nerve is another branch of the vagus nerve, and it innervates the muscles of the larynx other than the cricothyroid.
Many chemical processes, such as metabolism process, transform food and oxygen into substances that any human body uses to perform various functions, such as operating different organs by stimulating different nerve fibers. Metabolism is generally responsible for cell activity that regulates body energy. However, the chemical balance in each body as well as the rate of metabolism differs from person-to-person. One known primary factor responsible for these two aspects of the human body is inheritance of certain genes. Other factors include lifestyle, cultural, eating, and emotional issues that may adversely affect the chemical balance and the rate of metabolism, resulting in one or more metabolic disorders.
Certain chemicals called hormones produced by human organs (e.g., the endocrine glands, such as the thyroid gland), may cause metabolic or hormonal disorders. Hormones work with the human nervous system to control various organs and tissues for ensuring a smooth, effective and normal functioning of disparate parts of the human body in a cooperative manner. Specifically, when a hormone is released into the bloodstream, only a target part(s) of the human body is affected. The thyroid gland provides the thyroxin hormone, which controls metabolism rates in the body. Another endocrine gland, the pituitary gland, produces a thyroid-stimulating hormone that is regulated by a thyroid-releasing hormone. The metabolism rate is based on the amount of the thyroxin hormone secreted by the thyroid gland.
The thyroid gland processes hormones that are essential to body metabolism. A malfunction of the thyroid gland may result in increased or decreased amounts of the thyroxin hormones, which may cause metabolic disorders. Examples of metabolic disorders include increased appetite, weight loss, increased sensitivity to heat, diarrhea, muscle wasting, sweating, tremors, heart palpitations, thyrotoxicosis, toxic goiter, Graves' disease, and shortness of breath. Besides a drug regimen or surgical intervention, effective treatments for such diseases and disorders are fairly limited. Additionally, patient may build resistance to the drug regimens and/or surgical intervention may be unadvisable in some situations.